Field of the Invention
The present invention relates to the field of turbomachines, and in particular to a turbomachine element comprising an airfoil set with a plurality of airfoils that are offset from one another in a lateral direction, and vortex generator devices arranged upstream from said set in an axial direction that is perpendicular to said lateral direction.
Description of the Related Art
The term “turbomachine” is used in the present specification to designate any machine in which energy can be transferred between a fluid flow and at least one airfoil set, such as for example a compressor, a pump, a turbine, or indeed a combination of at least two of them. Such a turbomachine may have a plurality of stages, each stage normally having two airfoil sets, namely a movable blade set and a stationary guide vane set. Each airfoil set comprises a plurality of airfoils offset from one another in a lateral direction. Typically, the airfoils are arranged radially around a central axis. Thus, one such airfoil set forms a rotor, when it comprises a movable blade set, or else it forms a stator when it comprises a guide vane set. In such a stator, each vane is typically connected to an inner shroud by a proximal end or vane root, and to an outer shroud by an outer end or vane tip. The inner and outer shrouds are normally substantially coaxial, as shown for example in the French patent application published under the number FR 2 896 019.
Each airfoil presents a profile with a suction side, a pressure side, a leading edge, and a trailing edge, that in operation are subjected to the flow of a working fluid. In the description below, the terms “upstream” and “downstream” are defined relative to the normal flow direction of the working fluid. Under certain operating conditions, in particular in a high pressure compressor, it can happen that this flow separates from the suction side. More particularly at an airfoil root, a three-dimensional separation can occur, forming a “corner vortex”. This corner vortex is generated by an accumulation of particles having low kinetic energy in the corner formed between the suction side and the inner shroud forming the base of the airfoil. It gives rise in particular to a significant loss of efficiency in the compressor.
In order to reduce such corner vortices, proposals are made in international patent application WO 2008/046389 A1, to place vortex generator devices upstream from the airfoil set. In operation, the vortices generated by those devices deliver energy to the flow of the boundary layers adjacent to the suction side, in order to prevent local separation forming the airfoil corner vortex.
Nevertheless, using such vortex generator devices leads to certain problems. Firstly, it is preferable for the vortex generator device to offer as little resistance as possible to the flow of working fluid in order to minimize aerodynamic losses. Secondly, the device should preferably redirect the fluid flow locally in such a manner as to direct the vortex it generates towards the suction side. Typically, this implies that the vortex generator device must itself have an airfoil profile with, at its leading edge, an angle of attack that is small relative to the general flow direction of the fluid, while nevertheless presenting a large angle of incidence at its trailing edge. Nevertheless, it is difficult and expensive to fabricate a turbomachine element incorporating vortex generator airfoil devices, which are normally small in size.